A Test Boot, a Hump, and a Sensor in the Wrong Sump
A site is upgrading a 60-foot tank-to-dispenser run. The contractor installs 2-inch flexible double-wall piping in a sump-to-sump secondary containment arrangement: the primary pipe carries product, the annular space is monitored by one liquid sensor installed in the tank sump, and the interstitial space is intended to be open from the tank sump to each dispenser sump.
Two problems surface at inspection:
- A mechanical test boot remains installed on the primary pipe at the tank sump, and the interstitial sensor sits upstream of it.
- The run crosses a storm drain, so the contractor routed the piping in a 14-inch-high hump and then down to the dispensers. The dispenser sumps sit about 3 feet lower than the tank sump.
The contractor says 'the sensor is in the sump, so the whole annulus is monitored' and wants to backfill the trench.
1. Evaluate the contractor's claim. Where must the interstitial sensor be, and why?
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The claim is wrong. The test boot seals the annular space around the primary pipe at the tank sump. Any liquid escaping the primary pipe between the tank and the boot enters an annulus segment that is closed off from the sensor - the sensor sees nothing, and the system reports 'dry' while product sits in the annulus. The boot can also trap test water and produce false readings.
The sensor belongs at the low point of a continuously open interstitial space. Here that low point is the dispenser sump end, roughly 3 feet lower than the tank sump. Liquid in an annulus follows gravity, not the location of the sump closest to the tank.
Acceptable corrections:
• Remove the test boot after the installation test and confirm the annulus is continuous and unobstructed. • Or relocate the sensor downstream of the boot and provide a second sensor for the upstream segment. • Or use a manufacturer-certified arrangement that explicitly monitors each isolated segment.
Whichever is chosen, the annulus must drain to the sensor, the sensor must be product-rated and set at the correct height, and the sensor must be wired to the tank monitor so a disconnected sensor produces an alarm rather than silence.
2. What does the 14-inch hump do to leak detection and line handling, and what would you require?
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The hump is a trap in the interstitial path. Air or vapor collects at the crest and acts as a barrier: liquid leaking at the low end cannot rise past the hump to reach a sensor installed on the other side, and the annulus can hold product at the low end while the monitor reads dry. That defeats the entire 'liquid drains to the sensor' premise the design rests on.
The hump may also violate the flexible pipe manufacturer's bend-radius and support limits, and it creates a point where the primary line can trap product if the line is ever drained - complicating line clearing and tightness testing.
Require one of the following:
• Re-route to eliminate the hump - bore under the storm drain or use a sleeve. • If the hump is unavoidable, install an interstitial sensor at the low point on each side of the hump so each trapped segment is independently monitored.
Verify that slope is continuous toward the sensors, and document as-built elevations and sensor locations. 'The sensor is in the sump' is not a substitute for correct grading; on a sump-to-sump system the sensor location is a design decision that must appear on the as-built drawing and be proven during commissioning.
3. What tests are required before the dispensers are placed back in service?
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Before the dispensers go back into service:
• Primary line tightness test at 0.1 gph, or the state's new-installation standard (often a hydrostatic test at 1.5 times design pressure, or a 0.1 gph test), with the test boot removed or properly isolated so it cannot mask a leak. • Interstitial monitoring functional test - introduce liquid at the sensor or use the manufacturer's test port, confirm the tank monitor alarms, and confirm the alarm reaches the person responsible. • Secondary containment tightness - fill each sump hydrostatically and verify that the secondary pipe and all sump penetrations are liquid-tight from sump to sump. • Line leak detector test at 3 gph within one hour, plus the annual 0.1 gph line test. • Sensor verification - each sensor tested, correctly labeled, and shown on the monitor's point map. • As-built verification of slope, sensor locations, and bend radii against the manufacturer's instructions.
Record each test with date, method, instrument, tester, and result. A commissioning package with a missing or failed interstitial test is the item most likely to be cited at the first regulatory inspection.